Modern living is making the human race less intelligent as survival no longer depends on having the fittest genes, according to a new theory.

Slowly but surely, our intellectual and emotional abilities are being eroded because life is just too easy, it is claimed.

The slide backwards began long before the invention of reality TV, according to Professor Gerald Crabtree, a US developmental biologist from Stanford University in California.

He believes it started with the development of agriculture and supportive urban societies, which weakened the power of natural selection to "weed out" genetic defects damaging to intelligence.

Prof Crabtree bases his claim on a study of the frequency at which destructive mutations appear naturally in the genetic code.

Assuming that between 2,000 and 5,000 genes are needed for intellectual ability, he estimates that within around 3,000 years - or 120 generations - everyone will be carrying at least two mutations that are harmful to the brain.

Writing in the journal Trends in Genetics, Prof Crabtree said: "Expansion of the human frontal cortex and endocranial volume, to which we likely owe our capacity for abstract thought, predominantly occurred between 50,000 and 500,000 years ago in our prehistoric African ancestors, well before written language and before we had the modern voice box to produce sophisticated verbal language.

"Thus, the selective pressures that gave us our mental characteristics operated among non-verbal hunter-gatherers living in dispersed bands or villages, nothing like our present-day high-density, supportive societies."

Recent findings from neuroscience suggest genes involved in brain function are uniquely susceptible to mutations, he pointed out. Human intellectual and emotional capacity was at risk from a combination of less evolutionary pressure and large numbers of vulnerable genes.

On a more positive note, Prof Crabtree believes future technology will come to the rescue before the human race descends too far into stupidity.